It is known to leach with sodium carbonate solutions tungsten oxide (WO.sub.3) concentrates containing up to about 70% by weight of WO.sub.3, e.g., about 2% to 70% or 5% to 70% WO.sub.3. Low grade scheelite concentrates containing about 2% to 40% WO.sub.3 have been similarly leached.
A process illustrating the aforementioned type of leaching is disclosed in U.S. Pat. No. 4,351,808, the disclosure of which is incorporated herein by reference. According to this patent, scheelite concentrate is leached by forming it into a slurry in an aqueous sodium carbonate solution of initial concentration ranging from about 50 gpl to 200 gpl (grams per liter) at a Na.sub.2 CO.sub.3 /WO.sub.3 weight ratio of about 0.9 to 1.6 and then digesting the slurry in an autoclave at an elevated temperature ranging from about 180.degree. C. to 310.degree. C., e.g., about 200.degree. C. to 250.degree. C., preferably from about 200.degree. C. to 235.degree. C.
Optimum results are achieved by employing the Na.sub.2 CO.sub.3 concentration of the leach solution in substantially inverse relationship with the digestion temperature and in substantially direct relationship with the Na.sub.2 CO.sub.3 /WO.sub.3 weight ratio, the relationship selected being such as to effect the consistent dissolution of at least about 95% of the WO.sub.3 in the concentrate, preferably at least about 97%, and provide a pregnant liquor containing the WO.sub.3. By employing the foregoing relationship, the alkalinity can be controlled to levels favorable to the substantial inhibition of gangue minerals, such as gangue containing one or more of CaO, SiO.sub.2, calcium fluoride, etc. The presence of magnesium in the form of a magnesium-containing compound is very helpful in inhibiting substantially the dissolution of silica. The magnesium compound may be naturally present or deliberately added as dolomite. The amount of magnesium present may range from about 2% to 10% by weight of the concentrate, and preferably from about 4% to 6% by weight.
In a preferred embodiment, the initial soda ash concentration ranges from about 75 gpl to 175 gpl and more preferably from about 75 gpl to 150 gpl. Likewise, the Na.sub.2 CO.sub.3 /WO.sub.3 weight ratio may range from about 1 to 1.4.
The higher the digestion temperatures, the less is the amount of soda ash needed to achieve high tungsten extraction and the greater is the WO.sub.3 /Na.sub.2 CO.sub.3 ratio in the pregnant solution. Also the higher the initial concentration of Na.sub.2 CO.sub.3, the higher the weight ratio of Na.sub.2 CO.sub.3 /WO.sub.3 needed to obtain WO.sub.3 extraction in excess of 95%, e.g., at least about 97% or 98%. By controlling the Na.sub.2 CO.sub.3 /WO.sub.3 weight ratio, pregnant liquors containing up to about 130 or 140 gpl of WO.sub.3 are obtainable.
Generally speaking, the flotation concentrate contains residual amounts of flotation reagents which tend to interfere with subsequent hydrometallurgical processing. For example, the presence of flotation reagents can cause substantial foaming during hydrometallurgical processing. Foaming can lower the capacity of the processing vessels, or can cause hazardous conditions by overflowing such vessels. If the flotation reagents are carried on to subsequent process steps, such as solvent extraction, the flotation reagents can contaminate the organic phase causing substantial processing inefficiencies and often requiring periodic replacement of the organic extractant. In the processing of tungsten concentrates which contain significant amounts of molybdenum the presence of flotation reagents or other organic matter can reduce the effectiveness of the molybdenum removal operation.
Roasting the concentrate prior to treatment in the autoclave removes the carbonaceous organic matter. However, this method is expensive and, moreover, may generate arsenic fumes, if arsenic is present, or produce organic-fouled scheelite dust.
In U.S. Pat. No. 4,342,728, a process is proposed for removing the flotation reagents during digestion by carrying out the digestion in an alkaline solution under oxidizing conditions, the process comprising establishing a slurry of a sodium carbonate solution having sodium carbonate concentration between about 50 gpl and about 200 gpl and a scheelite concentrate in an amount to provide a Na.sub.2 CO.sub.3 /WO.sub.3 weight ratio of between about 0.9 to about 1.6, and heating the slurry to a temperature between about 230.degree. C. and about 320.degree. C. under an oxygen partial pressure of at least about 25 psi to digest the scheelite concentrate forming a pregnant sodium tungstate solution and to oxidize the organic matter.
We have found that we can avoid foaming either during or after the autoclave leaching, and protect the solvent extraction for contamination, by adding activated carbon to the tungsten-containing aqueous solution selective to the adsorption of flotation reagents, be it a slurry in the autoclave, or a tungsten solution following high pressure leaching, so long as the amount of carbon added (and the type) is sufficient to adsorb the flotation reagent or other carbonaceous organic material.